Bromostyrene polymers are known flame retardants for a variety of plastics, but have especially found use in the polyamide family due to their remarkable thermal stability. Commercially available homopolymers and functionalized copolymers of mixed mono-, di-, and tribromostyrenes, available from Great Lakes Chemical Corporation, are the products of choice for HTPA. The use of bromostyrene homopolymers in HTPA is described in numerous patents and publications, while functionalized bromostyrene-glycidyl(meth)acrylate copolymers in HTPA are disclosed in WO 02/24812 to Martens et al. of Dupont and in U.S. Pat. No. 6,414,064 B1 to Matsuoka and Sasaki of Kuraray.
HTPA incorporating bromostyrene polymers suffer from compromised melt flow. Improved molecular weight control for the bromostyrene polymers can provide increase the melt flow of the flame retarded HTPA composition, thereby improving the processability without compromise of flame retardancy. As taught in U.S. Pat. No. 5,304,618 to Atwell et al. of Great Lakes Chemical Corp., the molecular weight of the polybromostyrenes has typically been adjusted using 1-dodecanthiol as a chain transfer agent added during polymerization. For a period of time a homopolymer of mixed mono-, di- and tribromostyrenes having about 60 wt % bromine and a weight average molecular weight (Mw) of about 8,000 was commercially available from Great Lakes Chemical Corp. It was catalogued as “PDBS-10” and was marketed to the polyester and polyamide area. This low molecular weight/high melt flow polymer was produced by the use of a thiol chain transfer agent.
The use of α-methyl styrene dimer as a molecular weight regulator for free radical stryrenic polymerizations is known. See, for example, U.S. Pat. No. 5,559,200 to Suzuki et al. of Hitachi Chemical. Further, at a point many years after commercial introduction of PDBS-10 described above, JP 08-188622 to Horie and Kagawa of Tosoh Corp. restated that polymers of bromostyrene with an MW of between 1,000 to 10,000 have improved compatibility, flow, thermal discoloration resistance and electrical properties. The application notes that the benefits can be realized by solution polymerizing bromostyrene in the presence of a chain transfer agent to obtain an Mw of less than 10,000. While this publication does disclose that MSD may be used as the chain transfer agent, it does not indicate that any particular advantage is obtained in using MSD over the alkyl mercaptans (thiols) or alkyl halides that are described. Further, it does not teach the desirability in using MSD to produce polymers of bromostyrenes having an Mw of greater than 10,000.
While the known bromostyrene polymers serve as flame retardants for high temperature polymers, a need still exists for a polymeric flame retardant which improves processability high temperature polymers, particularly HTPA. An advantage found from the bromostyrenes disclosed is a reduced discoloration of the composition in the event it is processed at an elevated temperature for an extended period of time.